Br. H. Woodicard — On the Fossil Sirenia. 419 



with the figures given by Brandt/ and are also near to the ossicul^ 

 auditus of the Manatee. 



The sternum in Rhytina is a rather irregularly-shaped bone, not very 

 symmetrical bilaterally ; it has been figured by Brandt, and a portion 

 of one is preserved with the skeleton now in the British Museum. 

 It is a much thicker and stronger bone than the sternum of the 

 Manatee, but is similar in form. 



The scapula is somewhat convex externally, the inner concave 

 face fitting closely against the anterior ribs to which it was attached ; 

 the spinous process of the scapula is strongly developed. The glenoid 

 cavity is deep and circular and well fitted to the rounded head of 

 the humerus ; the humerus is short and very stout ; the radius and 

 ulna, which are also short, are anchylosed together at both extremi- 

 ties and incapable of any rotatory motion ; the olecranon is strongly 

 produced and curved, showing that the fore arm as well as the 

 humerus had considerable free lateral movement for the act of 

 swimming. 



The carpal bones and digits of the manus of Rlujtina are unknown ; 

 the digits were probably five, as in Manatus and Halicore, but the 

 thumb in the latter is rudimentary. Externally viewed, the fore 

 limb in Rhytina was fin-like, with no separate digits or nails visible ; 

 but Steller describes their extremities as thickly covered with short 

 bristly hairs. 



The skeleton is remarkable for the massiveness of the bones, 

 especially the great density of the ribs, which have the hardness of 

 ivory. This extreme density of the ribs is a character also shared by the 

 modern Manatee and Dugong, and *by the extinct Halitherinm and 

 Felsinotherium. There is a general absence of medullary cavities in 

 the bones. 



The great specific gravity of the bones no doubt assisted these 

 animals in keeping their bodies sunk beneath the surface of the 

 shallow waters in which they dwelt whilst feeding upon the marine 

 vegetation upon which they wholly subsisted. 



Although the normal number of cervical vertebrae maintained in 

 the Mammalia is usually seven, yet some variations are met with, as, 

 for example, in the " American Manatee," which has only six. Con- 

 versely in Bradypiis the number of the cervical vertebree is increased 

 to eight or nine. This is explained by the fact that the thoracic 

 vertebra in Bradijpus pass into the cervical region, while the 

 diminution to six in Choloepus and in the American Manatee is 

 similarly explained by the complete development of the rib of the 

 seventh cervical vertebra. 



Bhytina has been described by Steller as only possessing six 

 cervical vertebra, like the Manatee ; but Brandt correctly gives the 

 number as seven, and the specimen now in the Museum confirms 

 this determination. 



1 Brandt, " Syrab. Siren." Fasc. ii. pp. 8-10, Tab. ii. figs. 11-20. Mem. 

 Acad. Imp. Sci. St. Petersbourg, ser. vii. 1861. See also Claudius " On the 

 Organs of Hearing in Rhytina,'" Mem. Acad. Imp. Sci. St. Petersbourg, 1867, 

 vol. xi. No. 5, 2 plates. 



